QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,4100 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 20001, in Section 2.3, pages 27-33.
QKD systems were first developed in the form of a simple point-to-point connection between two users, called “Alice” and “Bob.” Nowadays, QKD systems are being considered as part of QKD-based networks that include multiple users as nodes in a dedicated QKD-system network, or multiple users incorporated into an existing classical telecommunications network.
The extension of QKD from point-to-point communication to network-type communication introduces some logistical issues. For example, QKD-based networks require the multiple users to have a process for establishing a common “quantum key” between any two users prior to their communicating with each other, even if they have never communicated with each other and/or even if they do not have a direct communication link. Furthermore, in view of the fact that a given user will often need to selectively communicate with one or just a few of the total number of users, there needs to be a way to authenticate the users with whom they are communicating to ensure their message is shared with a party intended to receive the message. In addition, quantum cryptography employs symmetric keys key rather than public keys key (asymmetric) cryptography used with public key infrastructure (PKI) cryptography.